In recent years, electronic balances have become common in the field of precision weighing. Those balances employ a transducer which produces an analog voltage related to a weight placed on a weighing platform connected to the transducer. The analog voltage is converted to a digital signal which is manipulated by a signal processor. Among other things, the digital signal is suitable for use by an apparatus which displays a number related to the weight placed on the platform.
The transducer and the weighing platform are subject to a wide variety of disturbances. Those disturbances can result in an unstable display of weight fluctuations. These fluctuations make it nearly impossible to accurately determine the weight on the platform. Accordingly, various analog filtering techniques have been proposed to alleviate this problem. Various active and passive low pass analog filters have been used to prevent disturbances from affecting the weight display. These filters have been unsatisfactory because adequate analog filtering is costly and analog filtering does not lend itself well to changing the filtering characteristics based on operational conditions of the balance and the environment in which the balance is used.
Another approach is digital filtering which has been prompted by the proliferation of microprocessors and digital circuitry in the weighing industry. Hanson et al. U.S. Pat. No. 4,139,070 ("the Hanson patent") suggests a digital filtering technique which takes a moving average of the most recent eight weight samples to overcome this problem. This technique is unsatisfactory because the display is slow to respond to changes in weight on the platform caused by adding or removing weight from the platform. It also does not permit changing the level of filtering to accommodate environments having differing disturbance levels and to accommodate the different operational modes of the balance, such as whether or not the weight receiver is stable and the accuracy and resolution to which the scale displays weight-related data. Gumberich et al. U.S. Pat. No. 4,328,874 addresses this problem but only suggests a simplified system for manually changing the number of weight samples that are averaged. Although this is suitable in some circumstances and is a significant advance over other approaches tried in the past, it has been found that additional flexibility is needed for the multitude of environments and operating conditions encountered by today's electronic balances. A stable display with good response time is also needed.
Another attempt to solve the problems introduced by disturbances of the weighing platform involved a digital filtering technique of automatically changing the number of samples that were averaged, such change having been based upon whether the display was stable. If the display was stable, the filter would take a straight rolling average of a first predetermined number of the most recent samples. When the most recent sample differed from the display by more than a predetermined amount, thus indicating that the balance was unstable, the number of samples that was averaged was automatically reduced to a second predetermined number. The second predetermined number was determined by a manually selected averaging level similar to the one disclosed in the Gumberich patent. When the balance again became stable, i.e. the most recent sample differed from the display by less than the predetermined amount, the number of samples that were averaged was increased by one for each successive display update that the balance was stable until the number of samples in the average was returned to the first predetermined number where it remained until the balance again became unstable.
Applicant has found a way to improve upon this arrangement. Applicant has developed a digital filter useful in an electronic balance which is not only able to take into account the varying operational conditions of the balance and the changing environments in which the balance is used but also is able to provide a stable weight display with good response time.
In most cases, the weight on the platform is not a linear function of the analog output of the weight transducer. The signal processors in the prior balances have included a linearization circuit in an attempt to obtain a signal linearly related to the weight on the platform. For example, the Hanson patent refers to a routine for linearizing the output of the weighing system in accordance with the equation WT'=WT+K(WT.sup.2) where WT is the output of the weighing system, WT' is the linearized weight signal, and K is a linearity constant. The linearity constant is empirically determined for each transducer and loaded into the processor. Such empirical determinations are made with special test equipment under carefully controlled conditions before the transducer is assembled into a balance.
Recently, balances have been suggested which contain circuitry which calculates the linearity constants for the transducer directly from the output of the transducer after it has been assembled into the balance. Thus, there is no need to separately measure the linearity constants of each transducer before it is put into the balance and then to manually load those constants into memory. However, the transducers, and the weighing platforms to which they are connected, are subject to the same disturbances alluded to above, which can result in inaccurate data from the transducer output to the circuitry calculating the transducer linearity constant. Thus, unreliable data from the transducer can be filtered as has been done in the past, but as discussed above, insufficient attention has been directed to the apparently mutually exclusive goals of stability and response time, both goals to be achieved at a reasonable cost.
Accordingly, there is a need for a filtering arrangement in electronic balances, especially those employing linearization circuitry which calculates the transducer linearity constants from the output of the transducer installed in the balance using the balance's own circuitry.